Heat treatment of formed aluminum alloy products

ABSTRACT

A process of producing a shaped article suitable for use as an automotive body panel intended for finishing by painting and, if necessary, baking. The process comprises obtaining a sheet article made of an aluminum alloy of the 2000 or 6000 series in a T4 or T4P temper and that exhibits an increase in hardness after painting and optionally baking, shaping the sheet article by forming to produce an intermediate shaped article, and subjecting the intermediate shaped article to a thermal spiking treatment prior to painting and optionally baking. The thermal spiking treatment involves heating the intermediate shaped article from ambient temperature to a temperature in a range of 150 to 300° C. with or without holding at that temperature for a period of time to enhance the increase in hardness. The process may also include the painting and optionally baking step. The invention includes the shaped articles, either prior to or after painting and optionally baking, produced by the process. The invention makes it possible to provide shaped articles that develop good hardness when used as automotive panels and the like, and may thus make it possible to reduce the gauge (and therefore weight) of those articles. This can be done without having to modify conventional procedures of casting and rolling to gauge to produce coiled sheet products.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority right of our U.S.Provisional patent application Serial No. 60/134,372, filed May 14,1999.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] This invention relates to a heat treatment process for shapedarticles, particularly those suitable for use in the fabrication ofautomotive body panels. More particularly, the invention relates to sucharticles made from aluminum alloy sheet material that exhibits animprovement of hardness after painting and baking operations have beencarried out.

[0004] II. Description of the Prior Art

[0005] Aluminum alloy sheet is being used more extensively nowadays as astructural and closure sheet material for vehicle bodies as automobilemanufacturers strive for improved fuel economy by reducing vehicleweight. Traditionally, aluminum alloy is either direct chill cast toform ingots or continuous cast in the form of a thick strip material,and then hot rolled to a preliminary thickness. In a separate operation,the strip is cold rolled to the final thickness and wound into coil. Thecoil must then undergo solution heat treatment to allow strengthening ofthe formed panel during painting and baking (steps usually carried outon shaped automotive parts by vehicle manufacturers or others—alsoreferred to as the paint bake or paint cure).

[0006] Several aluminum alloys of the AA (Aluminum Association) 2000 and6000 series are usually considered for automotive panel applications.The AA6000 series alloys contain magnesium and silicon, both with andwithout copper but, depending upon the Cu content, may be classified asAA2000 series alloys. These alloys are formable in the T4 or T4P temperconditions and become stronger after painting and baking. Good increasesin strength after painting and baking are highly desirable so thatthinner and therefore lighter panels may be employed.

[0007] It is highly desirable that the alloy sheet, when delivered tothe manufacturer, be relatively easily deformable so that it can bestamped or formed into products of the required shapes withoutdifficulty and without excessive springback. However, it is alsodesirable that the products, once formed and subjected to the normalpainting and baking procedure, be relatively hard so that thin sheet canbe employed and still provide good dent resistance.

[0008] To facilitate understanding, a brief explanation of theterminology used to describe alloy tempers may be in order at thisstage. The temper referred to as T4 is well known (see, for example,Aluminum Standards and Data (1984), page 11, published by The AluminumAssociation) and refers to alloy produced in the conventional manner,i.e. without intermediate batch annealing and pre-aging. This is thetemper in which automotive sheet panels are normally delivered to partsmanufacturers for forming into skin panels and the like. Material thathas undergone an intermediate batch annealing, but no pre-aging, is saidto have a T4A temper. An alloy that has only been solution heat-treatedand artificially aged to peak strength is said to be in the T6 temper.Material that has undergone pre-aging but not intermediate batchannealing is said to have a T4P temper, and material that has undergoneboth intermediate annealing and pre-aging is said to have a T4PA temper.T8 temper designates an alloy that has been solution heat-treated, coldworked and then artificially aged. Artificial aging involves holding thealloy at elevated temperature(s) over a period of time. T8X temperrefers to a T8 temper material that has been deformed in tension by 2%followed by a 30 minute treatment at 177° C. to represent the formingplus paint baking treatment typically experienced by formed automotivepanels.

[0009] An objective has been to provide a good “paint bake response”,i.e. a significant difference in hardness between the T4/T4P temper andthe final T8X temper.

[0010] In the past, attention has been directed to steps carried out onthe alloy sheets before the step of shaping the alloy sheets intoproducts. For example, in U.S. Pat. No. 5,728,241 issued on Mar. 17,1998 to Gupta et al., assigned to Alcan International Limited, a processof producing aluminum sheet of the 6000 series is described having T4and T8X tempers that are desirable for the production of automotiveparts. The aluminum alloy sheet material is subjected before shaping tosolution heat treatment and quenching and then, before substantial agehardening has taken place, the sheet material is subjected to one ormore heat treatments involving heating the material to a peaktemperature in the range of 100 to 300° C., holding the peak temperaturefor a period of time of less than one minute and then cooling the sheetmaterial.

[0011] Similarly, in U.S. Pat. No. 5,616,189 issued on Apr. 1, 1997 toJin et al., assigned to Alcan International Limited, a process isdisclosed that involves subjecting a sheet product, after cold rolling,to a solutionizing treatment (heating to 500 to 570° C.) followed by aquenching or cooling process involving carefully controlled coolingsteps to bring about a degree of “pre-aging.” This procedure results inthe formation of fine stable precipitate clusters that promote a fine,well dispersed precipitate structure during the paint/bake procedure towhich automotive panels are subjected, and consequently a relativelyhigh T8X temper.

[0012] While such approaches have met with success, they requiremodification of the traditional process for forming aluminum alloy sheetin strip form. This is inconvenient and may require expensivemodification of existing fabrication equipment. Moreover, the disclosedprocesses involve rather careful temperature control that can bedifficult or expensive to achieve.

[0013] It would be more convenient to be able to treat products made ofaluminum alloy sheet at in some way after they have been formed intodesired shapes. This is convenient because such products must anyway behandled and prepared for painting and baking, so additional steps atthis point are easily arranged.

SUMMARY OF THE INVENTION

[0014] An object of the invention is to provide a process of producing ashaped article of enhanced hardness response without modification of aconventional procedure for produced aluminum sheet material in T4 or T4Ptemper.

[0015] Another object of the present invention is to provide a solutionheat treated aluminum alloy product that exhibits a good hardnessresponse during shaped article formation and finishing.

[0016] Yet another object of the invention is to produce a formedproduct from an aluminum alloy sheet material that has a low yieldstrength in T4 temper and a high yield strength in T8X temper.

[0017] According to one aspect of the invention, there is provided aprocess of producing a painted shaped article, comprising:

[0018] obtaining a sheet article made of an aluminum alloy of the 2000or 6000 series in a T4 or T4P temper; shaping the article to form ashaped article; subjecting the shaped article to a thermal spikingtreatment involving heating the shaped article temporarily to a peaktemperature in a range of 150 to 300° C.;

[0019] applying paint to the article to form a painted shaped article;

[0020] and, if necessary to further enhance hardness of the paintedshaped article and/or to cure the applied paint, baking the article at atemperature of at least about 177° C.

[0021] The term “thermal spike treatment” means a step in which thearticle is quickly raised in temperature from ambient (or othertemperature at which the sheet material may be heated on the parttreatment line) to a predetermined maximum temperature and is thenquickly cooled or allowed to cool with or without providing a holdingperiod at the peak temperature.

[0022] The term “shaped article” includes any article obtained fromsheet material for use in fabricating an article or component. The termmay include a flat article simply cut from the sheet material, but oftenrefers to a non-planar article produced by a bending or stamping step,e.g. for the production of an automobile fender or door. The term doesnot include unformed or uncut sheet material of indefinite length, e.g.coiled sheet produced directly from ingots or cast strip.

[0023] The present invention may be carried out with any precipitationhardening aluminum alloy of the AA2000 or AA6000 series, i.e. alloyscontaining Al—Mg—Si or Al—Mg—Si—Cu that are capable of exhibiting an agehardening response.

[0024] The invention also relates to a painted and shaped sheet articleproduced by the above process.

[0025] While it has been usual in the past to refer to the desiredincrease in hardness as the “paint bake response”, this term is becomingsomewhat less appropriate as fabrication procedures advance. What isimportant is that this increase in hardness (the hardness response)occur between the shaping step (cutting/forming/stamping) initiallycarried out on the sheet form of the shaped product, and the finishingof the shaped product for delivery to the automobile manufacturer or thelike.

[0026] In modern processes, there may not be a traditional paint bakestep as paints of lower setting temperature may be employed. In thepresent application, the term “hardness response” will consequently beused instead of the more conventional term “paint bake response.” Thisterm refers to the change in tensile properties of the material at theend of a finishing process involving painting and optionally baking,compared to the properties prior to shaping. In the present invention,this increase may occur partially or fully during painting and baking,or partially or fully before such painting and baking, i.e. during theheat spike treatment itself, as will be explained more fully below.

[0027] The advantages of the invention, at least in preferred forms,include the following:

[0028] (1) The thermally spiked sheet material parts (e.g. automotivepanels) acquire higher strength than those panels which have not beenthermally spiked.

[0029] (2) In some forms of the invention, the maximum hardness responsein the formed part can be obtained through a thermal spiking alonewithout relying on the paint cure process (or without providing a paintcure at all).

[0030] (3) The thermal spiking process, at least in some forms of theinvention, can be performed on a continuous basis in ovens typicallyused for paint cure processes. The process therefore may be integratedseamlessly into the conventional shaping and finishing processes ofparts formation, thus leading to convenience, efficiency and economy.

[0031] (4) The process provides an alternative possibility to acquirestrengths higher than those obtained from the T4P material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is graph illustrating a typical thermal spike treatment inaccordance with the invention;

[0033]FIG. 2 is a graph as explained in the Examples below, showing thevariation in yield strength (YS) of conventional AA6111-T4 with (a)prestrain; and (b) prestrain plus ½ hour at 177° C.; and

[0034]FIG. 3 is a graph as explained in the Examples below, showing thevariation in yield strength (YS) of conventional AA6111, heat treatedaccording to one form of the present invention, with (a) prestrain; and(b) prestrain plus ½ hour at 177° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] According to the present invention, at least in its preferredforms, in order to improve the hardness response of AA2000 or AA6000series automotive alloy sheet in the T4/T4P temper, an article createdfrom the sheet is subjected to a thermal spike treatment at atemperature in the range of 150-300° C. after shaping (e.g.cutting/forming/stamping). The treatment may either involve a thermalspike confined to the lower part of the temperature range (e.g. 150-225°C.), which then relies on hardening from a subsequent paint bake step,or may involve a thermal spike into the upper part of the temperaturerange (e.g. 225-300° C.), which does not require additional hardeningfrom a paint bake step (baking to the conventional temperature range maythen be avoided, if desired, although conventional painting and bakingis not harmful). This latter form of the invention is of specialinterest because, in the future as new paints are developed, paint baketemperatures are expected to fall below 160° C., a temperature at whichhardening effects occur too slowly to fully strengthen the shapedproduct during normal curing times.

[0036] Conventional 6XXX materials in T4 or T4P tempers contain largenumber of fine metastable clusters and zones uniformly distributedthroughout a metal matrix. In the conventional process, during the paintcure, some fine unstable clusters/zones re-dissolve in the metal matrix,while other improve the material strength due to age hardening. Theprocess of the present invention allows the alloy material to exhibit anenhanced aging response (hardness response), although the exactmechanism is not clear. Without wishing to be bound to a particulartheory, it is believed that thermal spiking between 150 and 225° C.dissolves some of the clusters and zones and increases the solutesuper-saturation of the matrix of the formed part. Consequently, theformed part softens slightly, but the hardness response duringsubsequent painting and baking is improved in comparison with theconventional material. It should be noted that the formed part does notsoften when the thermal spiking treatment is carried out at higherspiking temperatures. This is largely due to the fact that the enhancedaging process masks the softening caused by the cluster dissolution.Surprisingly, the dislocations produced during part forming do notinterfere with the precipitation process as normally expected. Thisobservation allows the thermally spiked panels to acquire the desiredenhanced strength during the paint cure.

[0037] To achieve the desired hardness response, thermal spiking totemperatures in the lower part of the range (e.g. 150 to 225° C.) may becarried out at relatively slow heating rates (e.g. about 1 to 70°C./minute), especially if the article is not held at the peaktemperature for any time and is merely allowed to cool (or is forcefullycooled) as soon as the peak temperature is achieved. The relatively slowheating rate is often found to be necessary to improve the subsequentpaint bake response; i.e. the desired improvement in hardness will oftennot materialize if the heating rate is any higher. As a consequence, theheating to the peak temperature in this form of the invention may taketoo long for the step to be incorporated into a continuous stamping andpainting line. A batch treatment is therefore required.

[0038] If the thermal spiking extends into the upper temperature region(e.g. above 225° C.), the heating rate may be quite rapid (e.g. 10 to280° C./minute), even if there is essentially no holding time at thepeak temperature. It is found that the desired increase in hardness willoccur whether the heating rate is in the lower part or the higher partof the range indicated above, but for the process to be incorporatedinto a continuous stamping and painting/baking line, the peak metaltemperature (PMT) must generally be reached within about one minute. Ifthe lowest ambient temperature likely to be encountered is 15° C., theeffective range for a continuous operation would likely be 210 to 285°C./minute, which is the preferred heating rate for the high temperaturethermal spiking treatment.

[0039] The period of time for which the temperature is maintained at thepeak thermal spike temperature may range from zero to any time that ispractical in the circumstances. From the metallurgical point of view,the longer the time at which the temperature is maintained, the betterit is for achieving a desirable hardness response. In practice theperiod is usually from zero up to about 5 minutes.

[0040]FIG. 1 is a graphic representation of a preferred thermal spikingstep showing the preferred PMT range, the overall heating rate range andthe preferred time range at PMT.

[0041] The invention is illustrated by the following Examples, which arenot intended to be limiting.

EXAMPLE 1

[0042] The invention was tested using a commercially produced AA6111material.

[0043] DC ingot 600×1600 mm double length of the AA6111 alloy containing0.72% Cu, 0.7% Mg, 0.6% Si, 0.25% Fe, 0.20% Mn and 0.06% Cr was cast ona commercial scale. The ingots were scalped 12.5 mm per rolling face,fully homogenized, hot rolled and cold rolled to the final 0.93 mmgauge, fully solutionized, rapidly cooled, naturally aged for ≧48 hoursand sampled for laboratory evaluation.

[0044] The paint bake response of the material was evaluated aftersubjecting it to a heat treatment according to the invention. Tensilesamples were pre-strained by different amounts to simulate a typicalforming operation, thermally spiked in a sand bed furnace at 240° C. andaged for 30 minutes at 177° C. The results are summarized in Table 1below. TABLE 1 Tensile Properties of the Samples, with and withoutUni-Axial Pre-Strains, Thermally Spiked at 240° C. in a LaboratoryFurnace YS @ Tensile Properties After Simulated Paint Cure (%)Pre-Strain (1/2 h @ 177° C.) Inventive, Conventional Inventive %Increase in Pre- After Material Material YS from Strain Conven- Spikingat YS UTS % YS UTS % Conventional (%) tional 240° C. (MPa) (MPa) El(MPa) (MPa) El Material  0 145 103 176 299 24.2 200 312 21.3 13.6  2 189151 219 306 22.2 250 324 19.2 14.2  5 228 189 253 318 19.9 281 334 16.811.0 10 265 222 287 334 17.5 302 342 15.4  5.2

[0045] The variation in yield strength (YS) of the pre-strained andartificially aged (½ hour at 177° C.) material for both conventional andthe inventive process are plotted in FIGS. 2 and 3, respectively, of theaccompanying drawings.

[0046]FIG. 2 shows that the paint bake response of the AA6111-T4material increased about 30 MPa due to aging for 30 minutes at 177° C.(simulated paint cure). A similar response is observed in pre-strainedmaterial, although the net yield strength (YS) in the 5 and 10%pre-strained product is slightly lower due to recovery. The yieldstrength (YS) of the thermally spiked material decreases about 40 MPafor all levels of pre-strain, although the paint bake response is about90 MPa, which is greater than their conventional counterparts (compareFIGS. 2 and 3). The 10% pre-strained material shows slightly less paintbake response, which is related to the loss of strength due to recovery.In general, it is clear from FIGS. 2 and 3 that the inventive processimproves the paint bake response of the material, with and without priorpre-strain, quite considerably. This means that the process can be usedto heat-treat the formed part according to the invention and enhancedpaint cure strength could be achieved.

EXAMPLE 2

[0047] The tensile properties of the samples sheared from threedifferent locations of a hood, formed from a T4P temper material, weredetermined in the as-received and artificially aged conditions. Table 2lists the results of the tests carried out in variety of conditions.TABLE 2 Yield Strength (MPa) of a Hood Outer at Different LocationsBefore and After Aging at Different Temperatures Samples Near CenterLine Cut (Longitudinal) As Formed Plus Aging None 30 min @ 140° C. 30min @ 150° C. 30 min @ 177° C. Location Actual Actual Expected ActualExpected Actual Expected Front 219 231 252 236 263 — 297 Middle 218 230248 236 262 — 296 Rear 219 230 249 236 263 — 296 Driver Side Middle(Transverse) Front 226 — — — — 277 304 Middle — — — — — 270 292 Rear — —— — — 263 285

[0048] It can be seen that the ageing response of the hood material isabout 20 MPa lower than expected from the laboratory simulationexperiments in all aging conditions. Table 3 compares the properties ofthe hood material with those subjected to thermal spiking at 240° C.according to the inventive process. TABLE 3 Mechanical Properties of aHood Outer and the Effect of Thermal Spiking Driver Side (TransverseDirection) As Formed + As Formed + PMT @ 240° C. + As Formed 1/2 h @177° ½ h @ 177° C. Thick % YS UTS YS UTS YS UTS Location mm Red^(n) MPaMPa % El MPa MPa % El MPa MPa % El Middle 0.97 3.0 218 309 19 267 348 18281 352 16

[0049] It is clear that the strength of the thermally spiked materialafter aging 30 for minutes at 177° C. is about 14 MPa higher than itsconventional formed and aged counterpart.

What we claim is:
 1. A process of producing a painted shaped article,comprising: obtaining a sheet article made of an aluminum alloy of the2000 or 6000 series in a T4 or T4P temper; shaping the article to form ashaped article; subjecting the shaped article to a thermal spikingtreatment involving heating the shaped article temporarily to a peaktemperature in a range of 150 to 300° C.; applying paint to the articleto form a painted shaped article; and if necessary to further enhancehardness of the painted shaped article and/or to cure the applied paint,baking the article at a temperature of at least about 177° C.
 2. Theprocess of claim 1, wherein said peak temperature is within the range of150 to 225° C.
 3. The process of claim 2, wherein said heating of theshaped article is carried out at a rate in the range of 1 to 70°C./minute.
 4. The process of claim 2, wherein said painted shapedarticle is subjected to said baking at a temperature of at least 177° C.to further enhance said hardness.
 5. The process of claim 1, whereinsaid peak temperature is within the range of 225 to 300° C.
 6. Theprocess of claim 5, wherein said heating of said shaped article iscarried out at a rate in the range of 10 to 280° C./minute.
 7. Theprocess of claim 5, wherein said heating of said shaped article iscarried out at a rate in the range of 210 to 285° C./minute.
 8. Theprocess of claim 5, wherein said baking at said temperature of at leastabout 177° C. is omitted.
 9. The process of claim 1, wherein said shapedarticle is allowed to cool immediately after it reaches said peaktemperature during said thermal spiking treatment.
 10. The process ofclaim 1, wherein said shaped article is maintained at said peaktemperature for a period of time during said thermal spiking treatmentbefore being allowed to cool.
 11. The process of claim 10, wherein saidperiod of time is up to about 5 minutes.
 12. The process of claim 1,wherein said thermal spiking treatment is carried out in a continuousheat treatment furnace.
 13. The process of claim 7, wherein said thermalspiking treatment is carried out as part of a continuous shaping andpainting process.
 14. A shaped article suitable for use as an automotivepart, said article having been produced by a process comprising:obtaining a sheet article made of an aluminum alloy of the 2000 or 6000series in a T4 or T4P temper; shaping the article to form a shapedarticle; subjecting the shaped article to a thermal spiking treatmentinvolving heating the shaped article temporarily to a peak temperaturein a range of 150 to 300° C.; applying paint to the article to form apainted shaped article; and if necessary to further enhance hardness ofthe painted shaped article and/or to cure the applied paint, baking thearticle at a temperature of at least about 177° C.